1. Field of the Invention
The present invention relates to a digital camera signal processing device and, more particularly, to a device for directly digitizing a signal of an image sensor and performing signal processing on the digitized signal.
2. Description of the Related Art
In the field of analog signal processing, particularly so-called camera signal processing, different signal processing methods have conventionally bee utilized for different arrangements and structures of color filters for image sensors and different read-out methods therefor.
FIG. 1 shows an example of the arrangement of a vertical stripe system of Ye (yellow), G (green) and Cy (cyan). This system has a stripe structure in which each stripe consists of color filter elements of Ye, G and Cy which are respectively arrayed in three vertical columns. An image sensor (CCD) 200 transfers the signals of the respective color filter elements on a line-by-line basis in an interlaced manner to a CDS circuit 201. The CDS (Correction Double Sampling) circuit 201 extracts a difference signal from the input signal by effecting double-clamping, thereby eliminating noise which is called 1/f noise. An AGC (automatic gain control) amplifier 202 for sensitivity compensation raises its gain when the level of the signal from the image sensor 200 is low. Thus the AGC amplifier 202 outputs a sequential signal of Ye, G and Cy. A sample-and-hold circuit 203 separates the sequential signal of Ye, G and Cy into individual Ye, G and Cy signals. The signals sampled and held in the sample-and-hold circuit 203 form a three-phase signal in which the Ye, G and Cy signals differ in phase by 120.degree.. The Ye, G and Cy signals thus separated are subjected to the following processing as color signals. Since Ye=R+G and Cy=B+G, R and G are calculated from R=Ye-G and B=Cy-G. A matrix circuit 206 performs processing for R=Ye-G and B=Cy-G and produces a three-phase signal of R, G and B. The RGB signals are subjected to white balance adjustment in corresponding white balance circuits (WB) 207 on the basis of a signal which is obtained from an external-light measuring sensor 215 and associated circuit elements. The outputs of the white balance circuits 207 are subjected to gamma correction in the respective gamma correction circuits 209 and are then converted into color-difference signals B-Y and R-Y in a chroma matrix circuit 210. The color-difference signals B-Y and R-Y are respectively passed through low-pass filters 212 and 213 into an encoder.
In the meantime, processing of luminance signals is performed in the following manner. The separated Ye, G and Cy signals are applied to a level balance circuit 204, which in turn corrects the level variations or the like of the Ye, G and Cy signals which may result from variations in the characteristics of the color filter elements of Ye, G and Cy. Then, the thus-corrected signals are again converted into a sequential signal of Ye, G and Cy by a switch 205. The sequential signal outputted from the switch 205 is band-limited by a low-pass filter 211 and inputted into the aforesaid encoder. A low-pass filter 214 is provided for taking out a suppress signal used to compress a false color which often occurs in a high-luminance portion when a complementary-color filter is used. The suppress signal obtained by the low-pass filter 214 is supplied to a suppress circuit in the encoder and the suppress circuit performs a color-removing operation.
FIGS. 2(a) and 2(b) show a representative example of a complementary-color mosaic system. In FIGS. 2(a) and 2(b), a depiction of the entire system as in FIG. 1 is omitted. The shown system is intended for a color filter arrangement which assumes the structure ##STR1## along an n-th line and the structure ##STR2## along an (n+1)-th line, as shown in FIG. 2(b). The signal from the image sensor of the system is a sequential signal which is read out in an interlaced manner, as in FIG. 1. First, this image sensor outputs a color signal from the n-th line, as in the form Mg+Cy, G+Ye, Mg+Cy and G+Ye, and then from the (n+1)-th line in a mixed form different from that of the n-th line, such as Mg+Ye, G+Cy, Mg+Ye and G+Cy. The RGB components of each of these signals are arranged as shown in FIG. 2(b), that is to say, a BGBG component is obtained by passing the signal of the n-th line through a band-pass filter 301 and an RGRG component is obtained by passing the signal of the (n+1)-th line through the band-pass filter 301. The BGBG component and the RGRG component are subjected to detection, whereby color-difference signals of Cn=2B-G and Cn+1=2R-G are obtained.
In the meantime, processing of the luminance signals is performed in the following manner. The signals from the n-th line and the (n+1)-th line are passed through a low-pass filter 300, in which the signal from the n-th line is subjected to processing of Yn=(Ye+G)+(Cy+Mg)=2R+3G+2B, while the signal from the (n+1)-th line is subjected to processing of Yn+1=(Ye+Mg)+(Cy+G)=2R+3G+2B. The resultant signal is inputted into a process circuit 303. White balance adjustment is performed on the basis of the state of the color-difference signals. A low-pass filter 302 is provided for forming a signal for white balance adjustment, and the white balance adjustment is performed on the basis of this signal. As a matter of course, the low-pass filter 302 outputs different signals according to whether the input signal has been obtained from the n-th line or the (n+1)-th line. To cope with such different signal outputs, a 1H delay line 304 is provided and the two color-difference signals are converted into simultaneous signals by a switching circuit 305. The thus-obtained Y=2R+3G+2B and 2R-G, 2B-G are handled as a luminance signal and color-difference signals, respectively, and inputted into the encoder. Many other color filter arrangements are proposed, and the number of outputs from the image sensor is not necessarily limited to one. Some image sensors would provide interlaced outputs and others, noninterlaced outputs.
As is apparent from the foregoing description, the construction of a certain camera signal processing circuit is unique to a specific color filter arrangement which is prepared for a certain type of image sensor, and if a color filter of different arrangement is adopted for image sensors, it is necessary to change signal-processing ICs. As a result, the freedom of selection from image sensors is restricted when various products are to be developed and produced. In addition, in the case of a completely solid-state camera or the like in which digital data is recorded in a semiconductor memory, different makers adopt different color filter arrangements. Therefore, if compatibility with various image sensors is to be assured, it is inconveniently necessary to record a sensor output temporarily as separate signals, a luminance signal and color-difference signals.
To cope with the above-described problems, a method for realizing general-purpose camera signal processing has been proposed. In this method, image data formed on an image sensor is temporarily stored in a memory and is subsequently converted into pixel data in an n.times.m window. Then, conversion from the n.times.m pixels into Y (luminance) data, R (red) data, G (green) data and B (blue) data is performed through matrix computations, and identical processing is subsequently performed on the respective data. However, in such an example, for the matrix computations, it is necessary to program information indicative of what color is present in what position of the window, and it is also necessary to use means for scanning the window on the memory. This leads to the problem that a large circuit scale is needed. If there is no memory, a system utilizing the aforesaid method is unable to operate.